1. Field of the Invention
This invention relates to disc-type centrifuges, for separating solids-liquid slurries, which recycle thickened solids through the centrifuge. U.S. Pat. No. 3,799,431 discloses a centrifuge of this type.
2. Description of the Prior Art
In disc-type centrifuges it is often necessary to recycle thickened solids back into the centrifuge. In the absence of sufficient solids in the feed, excess liquid passes through the solids discharge nozzles and may cause equipment downstream of the disc centrifuge to malfunction.
It is known to use valves to adjust thickened solids recycle during centrifuge operation. Typically, a needle valve is used at the bottom of the stationary casing which surrounds the centrifuge rotor, with an additional control valve upstream of the needle valve. Such arrangements are disclosed in U.S. Pat. Nos. 2,616,620; 2,958,461; 3,080,108; and 3,204,868.
Another approach to solids recycle is to provide a rotatable screw within the recycle orifice, to promote active recycle of thickened solids by impelling the recycle solids upwards into the rotor. The screw is partially submerged in a sump at the bottom of the separating chamber. This approach is disclosed in U.S. Pat. No. 2,928,592.
Yet another approach is to utilize an upwardly directed nozzle, at the bottom of the stationary casing, which works in combination with a stationary aspirating ring to urge solids material falling to the bottom of the casing to join solids being directed upwardly into the separating rotor. The aspirating ring is not adjustable. This approach is disclosed in U.S. Pat. No. 2,559,453.
Other patents disclosing centrifuges, which may be of background interest, are U.S. Pat. Nos. 1,492,695 and 3,536,254. Neither of these disclose solids recycle.
To appreciate the problems to be overcome in designing a centrifuge which utilizes solids recycle, the mechanics of fluid flow through a valve must be understood:
Pressure drop across a valve is a function of (1) velocity and (2) viscosity of material flowing therethrough. The valve openable orifice presents a restriction which converts pressure head associated with material entering the valve into velocity head associated with material passing through the valve restriction. The length of the valve openable orifice is quite short. This minimizes the effect viscosity has on pressure drop across the valve since viscosity affects pressure drop, by slowing material flow, to the extent that shear stresses are created in the material as the material contacts the surrounding wall. Thus pressure drop across conventional valves is primarily a function of velocity and hence flow rate of material flowing through the valve; viscosity has a small, yet measurable, effect on pressure drop and changes in viscosity do not greatly effect pressure drop. Correlatively, mass flow rate through a conventional valve is primarily a function of pressure drop across the valve; viscosity and viscosity changes have a small effect on flow rate in conventional valves.
Use of valves, as taught by the prior art, to regulate thickened solids recycle flow, is disadvantageous.
When a valve is positioned in the centrifuge thickened solids discharge line, an increase in viscosity of discharged thickened solids results in a small increase in pressure drop across the valve, which increases pressure in the centrifuge. This increased pressure causes increased recycle of thickened solids within the centrifuge, which causes the concentration and, correspondingly, the viscosity of discharged thickened solids to increase still further; this phenomenon escalates until no thickened solids discharge from the centrifuge or until pressure within the centrifuge becomes high enough to force a slug of thickened solids through the valve. In either event the centrifuge thickened solids discharge varies in both concentration and flow rate; this is not permissible since downstream equipment may be adversely affected. In a similar manner, even small decreases in viscosity of discharged thickened solids can result in impermissible variations in thickened solids discharge concentration and/or flow rate.
When a valve is positioned in the centrifuge thickened solids recycle line, even a small increase in viscosity of discharged thickened solids increases pressure drop along the path from the nozzle discharge to the recycle inlet; this path includes the recycle line and the valve. Since the recycle line is a pipe, most of the increased pressure drop occurs in the pipe with only a minor portion of the increased pressure drop occurring at the valve (due to the aforementioned pressure drop-viscosity characteristic of valves). The increased pressure drop reduces recycle thickened solids flow rate, which reduces concentration and hence viscosity of the thickened solids; a degree of internal control of solids concentration results. However since the entire pipe is viscosity-sensitive, the valve provides only minimal capability to control recycle flow as a function of viscosity. (Indeed, the control provided by the valve is so minimal that some designers dispense with the valve and depend solely on the control afforded by the pipe pressure drop). Furthermore, the viscosity-sensitive character of the recycle solids line severely limits maximum recycle-solids flow rate attainable therethrough; this in turn limits the capability of the centrifuge to produce solids discharge of preselected concentration which does not change with variations in input slurry flow rate and/or solids concentration. In a similar manner, even small decreases in viscosity of discharged thickened solids can result in impermissible variations in thickened solids discharge concentration and/or flow rate.
Another known approach is to provide a viscometer and a pump which operates in response to the viscometer. The viscometer senses viscosity of thickened solids output by the centrifuge and, when the sensed viscosity becomes too low, actuates the pump to divert some of the solids output back into the recycle feed. These arrangements are typically quite costly.
For optimal operation of internal recycle centrifuges of the type of which this invention relates, a recycle solids sump pressure head must be maintained independent of recycle solids viscosity. Restriction orifices, valves, and the like cannot maintain a recycle solids sump pressure head which is independent of recycle solids viscosity since such devices are viscosity-sensitive. (The above-presented discussion of viscosity-pressure drop characteristics of conventional valves is equally applicable to restriction orifices.) Pumps may be used to perform this function but become excessively expensive when coupled with requisite sump pressure measuring and pump control apparatus.
U.S. Pat. No. 3,536,254 discloses an adjustable weir in conjunction with a centrifuge. The weir is used to maintain water level in the centrifuge tank. The weir disclosed in the U.S. Pat. No. 3,536,254 consists of lengths of pipe which are viscosity sensitive, i.e. pressure drop along the pipe changes as viscosity of fluid flowing through the pipe changes. Thus U.S. Pat. No. 3,536,254 weir cannot maintain a constant pressure head when a material of variable viscosity, such as a solids-liquid slurry, flows through the pipes forming the weir. The U.S. Pat. No. 3,536,254 does not disclose a disc-type centrifuge of the kind to which this invention relates; there is no recycle of solids at the bottom of the centrifuge bowl in the apparatus shown in the U.S. Pat. No. 3,536,254. Moreover, the U.S. Pat. No. 3,536,254 does not suggest regulating the pressure head of a viscous slurry to thereby effect recycle of solids through an adjustable orifice in a disc-type centrifuge. The U.S. Pat. No. 3,536,254 controls water level within a stationary bowl to thereby control an oil-water interface level, when oil is being separated from water.